Temperatures are often measured with electrical temperature sensors. In particular, thermal resistors, known as RTD (Resistance Temperature Detector), such as PT10, PT100, PT1000, NTC30, NTC22, are used. They are based on the principle that a current flowing through the thermal resistor causes a voltage difference over the RTD, which is related to the temperature of the RTD in a monotonous way.
For measuring a temperature with an RTD, the RTD is exposed to the site which temperature is of interest at least until the RTD reaches thermal equilibrium. Afterwards a known current is conducted through the RTD. This current can be relatively small in order to minimize measurement errors arising from an ohmic heating of the material of the RTD. This is followed by measuring the voltage across the RTD. The resistance can be calculated from the known current and the measured voltage, which value is related to the temperature at the site of interest.
In practice, the RTD is often physically inaccessible. For example, the RTD might be placed deep in a caustic chemical bath arranged remotely from the measurement instrumentation. As a result, wire leads can be used to connect the RTD to a voltage measurement circuit. In such cases, the determined resistance is the sum of the RTD resistance and the wire resistance associated with the extended wire leads. The wire resistance might introduce measurement errors, especially for PT10, PT100 and PT1000.
One approach for eliminating the adverse effect of the connecting wire resistance is to apply a so-called four-wire connection. One pair of wires is used for voltage measurements only, whereas the other pair of wires conducts the measurement current. However, this solution can generate significant additional costs for the wiring.
Another approach for eliminating the adverse effect of the connecting wire resistance is to apply a so-called three-wire connection. According to this implementation, the thermal resistor is connected to the voltage measurement circuit by three connecting wires. The third wire helps to compensate the impact of the wire resistance. However, this three-wire connection involves use of a proper voltage measurement circuit.
A common approach to build measurement devices suitable for conducting the three-wire method is to use a dual constant-current source. However, the accuracy of such a circuit relies heavily on the two current sources being identical. To avoid the difficulties associated with obtaining two identical current sources, methods with a single current source are generally used.
Different solutions have been proposed for suitable voltage measurement circuits with a single constant-current source.
For example CN2692646 describes a circuit with a single constant-current source and an operational amplifier which serves as a subtraction unit. However, such circuits can be complicated and the measurement range of the circuit is relatively narrow. Therefore, adjustments of the circuit and the constant-current source, like switching or jumpering, are used for changing to different temperature ranges or to different types of thermal resistors.
On the other hand, simple circuits can only compensate the effect of the connecting wire resistance for a limited number of points of the measurement range, rather than fully compensating any measurement point over the whole measurement range.